The invention relates to a screw press, preferably for removing liquid from a material to be conveyed, comprising a screw shaft complete with a helical conveying flight mounted on the screw shaft and a housing shell surrounding it, where a transport channel is formed for the material to be conveyed between the screw shaft, the conveying flight, and the housing shell.
The conveyed material to be processed by the screw press is usually a solid-liquid mixture, for example industrially manufactured pulp fibres, from which the entrained liquid is to be extracted.
The principle of the pressing process effected by the screw press is sufficiently well known: A screw shaft with a conveying flight is mounted on bearings inside a cylindrical or conical housing shell. The housing shell has a filling port provided with a filling funnel and also an outlet port provided with an outlet nozzle.
Helical transport channels are formed between the housing shell, the screw shaft and the conveying flight, leading from the filling port to the outlet port and which narrow successively towards the area of the outlet port.
When the screw shaft is set in rotation, material to be conveyed that is fed to the screw press through the filling port is conveyed along the transport channels to the outlet port by the conveying flight mounted on the screw shaft and is compacted and dried by pressing continuously during this process.
Liquid removed from the conveyed material during this compacting process is directed through slotted or perforated liquid passages in the housing shell to a collecting device.
A screw press of this kind is known, for example from AT 412 857 B.
Particular problems have arisen with screw presses of the generic kind because the conveyed material transported in the transport channels causes constant abrasion to the screw shaft, and after a certain period in operation, the surface of the screw shaft is finally rubbed away leaving a polished surface. There is a tendency for the conveyed material, e.g. the pulp fibres, to stick to a screw shaft that has been smoothed in this way and thus to rotate more together with the screw shaft than was the case with the previously rough screw shaft.
Increased rotation of the conveyed material with the screw shaft leads to a lower throughput speed of the conveyed material through the conveying channels, which has to be compensated in turn by increased screw shaft speed and thus, higher energy input.
Furthermore, the conveyed material components adhering to the screw shaft cause uneven filling, or even plugging, of the transport channels.
A further undesirable consequence of the described tendency to adhere to the screw shaft is that liquid removal from the conveyed material—viewed in a comparison of the individual conveying channel sectors distributed along the length of the screw shaft—becomes less homogenous.
Several attempts have already been made to counteract this problem of screw shafts rubbed away to a smooth surface.
The methods put into practice included coating the screw shaft surface with a suitable substance, e.g. tungsten carbide, and thus maintaining a rough surface accompanied by a lower tendency to adhere to the screw shaft for a longer period. These coatings are sprayed or sinter-fused directly onto the screw shaft or onto a sheet metal element that is then secured to the surface of the screw shaft.
The disadvantage of these kinds of coating process is that they are very complicated and very expensive. The sustainability achieved by coating is still unsatisfactory, and practical experience has shown that the coating applied to the screw shaft was already worn after less than two years in the treatment of waste paper with a relatively high ash content, for example, and the screw shaft was rubbed away to a smooth surface again, thus making it susceptible to sedimentation of the conveyed material once again.
Due to the considerable effort involved in applying a coating to the screw shaft, however, screw shafts are also roughened again at regular intervals in a grinding process, so that the surface of the screw shaft is virtually restored to the condition it was in immediately after manufacture.
This roughening or reworking must be performed, however, at relatively brief intervals (in some cases every one to three months) and requires considerable manual effort. Since the screw press has to be shut down for this process, there is also a loss of production.
The problem thus addressed by the present invention is to avoid the disadvantages mentioned and to enable the screw shaft to remain free at all times of any conveyed material adhering to it by providing a non-stick surface structure for the screw shaft with a substantially longer life cycle.